Flegmatized explosive

Abstract: An explosive in semi-plastic, paste or slurry form, comprising at least one high explosive in molecular form and comprising or including at least one pulverulent component, which explosive further comprises a flegmatizing agent, including solid particles of a material more soft or deformable than the particles of said pulverulent component.

Description: The present invention relates to an explosive in semi-plastic, paste or slurry form, comprising at least one high explosive in molecular form and comprising or including at least one pulverulent component. A basic design problem for explosive compositions is to find an acceptable balance between safety and insensitivity to unintended ignition in manufacture, transport and handling of the explosive before blast and reliable ignition and detonation in the blast. Actions taken to affect one of the properties generally also affect the other. Intentional initiation is normally influenced by shock applied form a detonator or primer charge and it is desirable that the explosive is more sensitive to these influences than for example energy applied through friction, heat or electric discharges. It is further desirable that the sensitivity can be fairly easily varied as explosives used in larger volumes or bore-hole diameters need less sensitivity to obtain ignition and detonation than do explosives used in smaller volumes or narrower diameters. Many modern explosives such as emulsion or slurry explosives, based mainly on intimate mixtures of fuels and oxidizers rather than self-explosive compounds, offer numerous parameters for affecting sensitivity, including disintegration degree, interface between the phases and composition density. Sensitivity in manufacture is rarely a problem as an initiable explosive is only obtained in the last manufacturing steps. In contrast hereto classical explosives based on molecular high explosives are not as easily affected in respect of sensitivity since this property to a large extent is inherent in the molecular explosive itself. Safety and sensitivity aspects also tend to be included in larger parts of the manufacturing process for the molecular explosives. Molecular high explosives normally comes either in solid crystalline form, e.g. TNT, or in liquid form, e.g. liquid nitric acid esters such as nitroglycerines or nitroglycols, hereinafter generally referred to as "NG". From a rheological standpoint commercial explosives are preferred in particulate, paste or slurry form, rather than solid or liquid form. The crystalline explosives may be mixed with more liquid component to obtain a paste and NG is normally gelled and mixed with large amounts of solid components, such as oxidising salts. The presence of solid pulverulent components in a deformable explosive influences its sensitivity properties and its response to friction in particular. The ubiquitous liquid phase in paste and slurry type explosives often makes it especially difficult to affect sensitivity properly. Solid crystalline explosives have been given reduced initiability properties, hereinafter called "flegmatized", by additions on the particle surfaces or in the interstices between particles. Examples of additives for this purpose are melted wax or inert compounds. The additions act to generally inhibit mass and heat transfer between the particles, reducing not only sensitivity but also ignitability and overall reaction speed. In NG explosives any flegmatizing agent is normally added and dissolved in the gel obtained between the NG component and the gelling agent. The requirement for dissolution of the agent severely limits the number of possible compounds. The NG gel has a limited stability in the first place and tends to be less stable at the addition of further components. A chemical similarity is required between the agent and the gel components and generally various nitrated compounds have been used, some of which, such as mono and dinitrotoluene, represent a health hazard and in many instances are incompatible with common packing materials due to volatility or plastizising effect. As for crystalline explosives the additives act to reduce not only the deleterious sensitivity but also ignitability and reactivity in general. US-A-1,828,788 discloses a nitroglycerine explosive composition comprising wax impregnated wood sawdust, as a combustible substitute for sulphur, but which is stated not to reduce the total power or execution capacity of the explosive. US-A-3,345,224 relates to a dry-appearing explosive composition mixable in conventional dry mixing equipment, comprising particulate porous material having liquid absorbed therein, such that under extrusion pressure the liquid is expressed to provide a semi-solid or thixotropic consistency. These two prior art references do not discuss factors influencing sensitivity to mechanically induced initiation and do not relate to flegmatizing of explosives. Summary of the invention A main object of the present invention is to provide a flegmatized explosive without the abovesaid disadvantages. A more specific object is to provide an explosive containing a flegmatizing agent which is generally useful for compositions containing pulverulent components. Another object is to provide a flegmatized explosive with easily variable sensitivity. A further object is to provide a flegmatized explosive with maintained good ignitability and reactivity. Yet another object is to provide an explosive with reduced friction sensitivity. Still another object is to provide a flegmatized explosive of low cost. A further object is to provide an explosive flegmatized with non-hazardous components. These objects are reached by the characteristics set forth in the appended claims. Accordingly, the present invention provides an explosive composition in semi-plastic, paste or slurry form, comprising more than 10 percent by weight of at least one liquid nitric acid ester compound, gelled with nitrocellulose, at least one crystalline pulverulent component comprising an oxidizing salt, and a flegmatizing agent, characterized in that said flegmatizing agent comprises 0.5 to 10 percent, by volume of the composition, of solid particles of a material more soft or deformable than the particles of said crystalline pulverulent component, and in that the flegmatizing particles have a weight average particle size greater than the weight average particle size of the crystalline pulverulent component. The particles softer than the pulverulent components in an explosive containing a molecular explosive act to fulfil several of the postulated objectives. Without being bound by any theory, it is believed that the spotwise presence of particles in the explosive composition, contrary to a melted or distributed additive, contributes to a maintained high general reactivity and ignitability in the composition by avoided manipulation of the basic properties of the molecular explosive either in solid or liquid form. The unaffected reactivity provides stable ignition and detonation properties at broad temperature ranges. Yet, when subjected to mechanical stress the softer particles tend to yield first, thereby effectively dampening the effects of such stress and being particularly effective in counteracting local concentrations of energy release, which concentrations are otherwise unavoidable in a particulate mass under stress. It is further believed that the deformable nature of the particles are especially effective in damping out shear and friction stresses, hypothetically through a lubricating response to such stresses. Since the particle flegmatizing effect is believed to be mechanical rather than chemical the principles are applicable for a variety of explosives having the basic requirements stated. The flegmatizing effect can be easily varied through the amount of particles added. The solid character of the particles further serves to eliminate hitherto experienced problems caused by volatile or migrating additives. The additive cooperates well with explosives containing a liquid phase. As there is no requirement for dissolution of the agent any fluid explosive component is left unaffected and especially the rheology properties of NG gel based explosives are improved over known types. The same property further makes a broad range of materials useful for the purposes of the invention. Materials compatible with both hydrophilic and lipophilic compositions can easily be found as well as materials which are inert, harmless and without smell. The freedom of choice allows material selections providing secondary advantages, such as improved water resistance and reduced salt caking with hydrophobic additives. Further objects and advantages of the invention will be evident from the detailed description hereinbelow. Details of the invention The principles of the invention are mainly useful for explosives in semi-plastic, paste or slurry form, i.e. deformable explosives able to adapt to various forms during charging or otherwise, rather than rigid solid or molded explosives. The invention is applied for explosives comprising a gel phase and preferably pastelike compositions having a certain plasticity. The explosive may be in bulk form for direct delivery into bore-holes although certain additional advantages are obtained for packaged explosives, e.g. in tube or cartridge form of any size and dimension. The invention nay be used for explosives requiring primer or booster for initiation but is of particular value for more sensitive explosives initiable with a number 8 cap or less. It is of special value to utilize the invention for explosives otherwise with high sensitivity for ignition through mechanical stress, friction in particular. Although the invention nay have some general utility in explosive compositions, the invention comprises in particular molecular explosive, by which concept shall be understood both true molecular explosives, having fuel and oxidizer components in the same molecule, as well as other self-exploding combinations having the same mixing intimacy, such as co-crystallized fuel and oxidizer components and solutions of such components, all being characterized by the absence of any macroscopical interfaces between critical components. The explosive contains a true molecular explosive such as high explosives. Such an explosive may be in solid or liquid form. Solid explosives may be in amorphous form, such as melted and fast cooled molecular explosive, but are commonly and preferably in crystalline form. Typical but non-exhaustive examples are RDX, HMX, HNS, PETN and especially TNT. Liquid explosives in general may be solutions, preferably substantially purely between oxidizer and fuel, or preferably liquid true molecular explosives such nitroparaffins, e.g. nitromethane, nitroethane, nitropropane, tetranitromethane etc, but in accordance with the invention NGs are used. The enumeration includes combinations and mixtures within and between the groups. To take full advantage of the current advantages the amount of NG molecular explosive in the explosive composition should exceed 10 percent by weight and preferably exceed 12 percent by weight. The explosives for use with the invention are the NGs, i.e. liquid nitric acid esters, and especially glycerine trinitrate and in particular ethylene glycol dinitrate. These liquid, true molecular explosives are normally gelled or thickened with a gelling agent, which for example may be nitrated starch or, in accordance with the invention, nitrocellulose. In order to obtain a non-liquid the composition further contains substantial amounts of solids, which may be an inert filler such as silica guhr, but in accordance with the invention includes an oxidizing salt, to be further explained below, optionally together with a further solid fuel, such as flour, wood meal or carbon or liquid fuel, such as partially nitrated paraffins or aromatics. The composition may contain further additives in smaller amounts for specific purposes, such as flame-cooling salts, colouring or buffering agents. Depending mainly on the amount ratio between NG gel and solids, the product may be obtained in substantially pulverulent form, with NG gel amounts up to about 15 percent by weight, semi-plastic form with amounts between about 13 to 20 percent and substantially plastic form with amounts between about 15 and 60 percent, preferably between 18 and 30 percent by weight. The present invention is preferably used with the described semi-plastic and plastic or paste varieties and especially the conventional NG type, having an oxidizer salt as the main solid component and gelled NG in an amount sufficient to provide the desired rheology. For mixtures with other explosives or components, a sufficient total liquid, fluid or gel content should be present to give the desired semi-plastic, paste or slurry rheology. In such instances the above given amount values for NG are about equally applicable, in percent by volume, for the total fluid content for obtaining corresponding rheology properties. The current explosive shall contain at least one pulverulent component, other than the flegmatizing agent. This pulverulent component may be of various kinds depending on the basic nature of the explosive applied. The component can be an inert filler such as a clay, an absorbing filler such as guhr, a void-containing density reducing filler such as mineral or plastic microspheres or foamed polystyrene beads. Preferred components are the abovementioned solid explosives and, in accordance with the invention, oxidising salts. Common oxidizing salts are inorganic nitrates and optionally also perchlorates. Ammonium nitrate is generally preferred in addition to alkali or alkaline earth metal nitrates and perchlorates. The pulverulent component is crystalline rather than amorphous in character. The explosive may contain a mixture between several of said pulverulent components or with other such components in larger or smaller amounts. The total solids content in the explosive may be rather high, such as above 20 percent by volume, better above 40 percent and preferably above 60 percent by volume. Plastic explosives may have sufficient solids to provide the desired rheology. For the present purpose "pulverulent component" refers to the dominating, volume based part of the solids in the composition or the harder, solid component in mixtures of several volume significant parts. The flegmatizing agent comprises, and preferably consists of, particles more soft or deformable than the particles of the pulverulent component. The particle condition requires that the material shall remain solid and substantially retain its dimensions at all intended use temperatures up to initiation. Preferably, the material has a melting or softening temperature above 40 and more preferably above 60 degrees centrigrade. It also requires that the material is substantially chemically stable or inert in the composition and has a sufficently low swellability or solubility in the explosive not to change materially over its intended use time. The soft or deformable character of the particles can be obtained in several ways. Soft crystalline materials such as talc or graphite can be selected. Crystalline materials can also be soft through a weak agglomeration between harder particles or through thin walls in porous materials. Preferably, however, the material is deformable with maintained phase structure, by comprising a major part of an amorphous or semi-liquid phase, giving most pronounced lubricating properties. The phase may contain non-liquid components such as microcrystals but is preferably substantially homogeneous. The phase may comprise an inorganic material such as gels or water-glasses, especially when hydrophilic materials are desired, but organic materials are generally preferred. The selection depends, i.a., on the overall hydrophilic or lipophilic character of the explosive compositions. In lipophilic or oil containing explosives generally hydrophilic materials can be selected, such as carbohydrates or polymers with hydrophilic substituents. In generally hydrophilic explosive compositions, such as NG explosives, by preference the material can be hydrophobic. Oligomers or polymers, possibly with softening plastizisers, can be used. Simple, pure or substantially pure hydrocarbons, especially the saturated types, can be used. Preferred materials are those generally termed "waxes", such as acid waxes, ester waxes, oxazolin waxes and especially the hydrocarbon waxes such as paraffin waxes or petroleum waxes. Although there is no absolute correlation between the softness of a material and its softening or melting temperature, a general indication is that the melting or softening temperature should be below 250 degrees, better below 200 degrees and preferably below 150 degrees centigrade. The softness condition requires that if the mixture between pulverulent component and the solid particles is subjected to compression, shear or other mechanical stress the particles shall yield, deform or float clearly before the particles in the pulverulent component are deformed. Ammonium nitrate is a common component of explosives and taken as a relative reference it is preferred that the particles are clearly more soft or deformable than crystals of ammonium nitrate salt. In absolute terms it is preferred that the particles have a Moh's hardness below 3, preferably below 2 and most preferably below 1, at the design temperature. The lower limit is determined by the requirement for stable particulate properties at use conditions, as described. The flegmatizing agent particles have a weight average particle size clearly greater that the weight average particle size of the particles in the pulverulent component or solid content generally, whichever is greater, although soft particles such as wood meal or flour can be disregarded in the average calculation. Preferably the stated average is 2 times greater and more preferably 5 times greater. In absolute terms said average particle size may be between 0.01 and 5 mm and preferably between 0.1 and 1 mm. The amount of flegmatizing agent included in the explosive depends on the desired degree of sensitivity reduction. The additive is surprisingly effective and only limited amounts are needed. The amounts shall be clearly less than the rest of the solids content in the composition on a volume basis, e.g. less than 1/5 and preferably less than 1/10 of said amount. Preferably the amount on a volume basis is also clearly less than the pulverulent component amount, e.g. less than 1/5 and preferably less than 1/10 of said amount. The upper limit is determined mainly by the decreasing contribution to sensitivity reduction, the dilution effects in the explosive and, in case of combustible additives, the additional fuel value introduced. In absolute terms a suitable range is between 0.1 and 15 percent by volume and preferably between 0.5 and 10 percent by volume. Other flegmatizing agents can be used in conjunction with the current agent, e.g. to partly influence sensitivity also in a traditional manner. Any known flegmatizing agent can be used, such as melted or liquid components distributed in the composition. Liquid components are generally preferred. Suitable hydrophobic liquid components are oils and paraffin oil in particular, which adds to water resistance and favourable viscosity properties. A suitable hydrophilic additive is water. The amount of such additives should be kept low, suitably below 5 percent by weight and preferably below 2 percent. As a general rule the amount of such additional agents should be kept lower than the amount of particulate agent according to the invention. The present explosives can be manufactured with substantially the same methods as similar explosives without the agent. The agent can be introduced in a simple mixing step, preferably together with other solid components to optimally utilize the flegmatizing effect during the manufacture. Only high mechanical stress under high temperatures can inactivate the agent, which conditions are rarely present in the explosives manfacture. Examples In the following examples the same general manufacturing method was used. A stream of pure nitroglycol, separated from a water emulsion after a nitration step, is weighed and delivered to a mixing vessel having screw agitating means. A weighed amount of nitrocellulose is charged into the vessel together with optional wetting and colouring agent additives and dinitrotoluene, when present (Example 1), slightly heated above its melting point. The mixture is agitated a couple of minutes until a gel is formed. If paraffin oil is present this is then added followed by the addition of the solid components, wood meal, flour, ammonium nitrate salt and the paraffin wax component. Optionally, extra water is sprayed into the mixture from a nozzle simultaneously with the salt addition. The mixture is kneaded about 3 to 4 minutes or until a substantially homogeneous mixture is obtained. The falling weight impact sensitivity is measured by placing a sample of the explosive of about 2 mm diameter and 1 mm thickness on an abrasive mat in a cylindrical cavity, substantially larger than the sample, in a steel body, inserting a cylindrical steel piston into the cavity in such a position that it is kept clear from the top surface of the sample. A 1 kg weight is then dropped from various heights and allowed to hit and drive the piston to the cavity bottom under compression of the sample. The minimum height for sample ignition is determined. Example 1 A first, comparative, composition was prepared with dinitrotoluene as a conventional flegmatizing means. The composition had the following ingredients, expressed in parts by weight: Nitroglycol 23.0 Dinitrotoluene 7.1 Nitrocellulose 1) 1.6 Wood meal 2) 1.7 Flour 3) 0.7 Ammonium nitrate 4) 65.9 The composition had a falling weight impact sensitivity of about 24-25 cm. Example 2 A composition was prepared with a particulate paraffin wax as flegmatizing agent. The composition had the following ingredients, expressed in parts by weight: Nitroglycol 26.0 Nitrocellulose 1) 1.3 Wood meal 2) 1.3 Flour 3) 2.0 Ammonium nitrate 4) 66.8 Paraffin wax 5) 1.8 Paraffin oil 6) 0.8 The composition had a falling weight impact sensitivity of about 25 cm. Example 3 A composition was prepared with a particulate paraffin wax as flegmatizing agent. The composition had the following ingredients, expressed in parts by weight: Nitroglycol 26.0 Nitrocellulose 1) 1.2 Wood meal 2) 1.5 Flour 3) 2.1 Ammonium nitrate 4) 66.5 Paraffin wax 5) 2.2 Water 0.5 The composition had a falling weight impact sensitivity of about 24 cm. Example 4 A composition was prepared with a particulate paraffin wax as flegmatizing agent. The composition had the following ingredients, expressed in parts by weight: Nitroglycol 26.0 Nitrocellulose 1) 1.2 Wood meal 2) 1.4 Flour 3) 2.1 Ammonium nitrate 4) 66.5 Paraffin wax 5) 2.2 Water 0.6 Wetting agent 7) 0.02 The composition had a falling weight impact sensitivity of about 24 cm. Example 5 A composition was prepared with a particulate paraffin wax as flegmatizing agent. The composition had the following ingredients, expressed in parts by weight: Nitroglycol 26.0 Nitrocellulose 1) 1.3 Wood meal 2) 1.5 Flour 3) 1.5 Ammonium nitrate 4) 67.7 Paraffin wax 5) 2.0 The composition has a falling weight impact sensitivity of about 25 cm. Notes 1. Nitrocellulose, Bofors Dynamite NC, 12.2 to 12.5% nitrogen. 2. Wood meal, having a particle size of about 250 microns. 3. Rye flour, having a particle size less than 100 microns. 4. Particulate ammonium nitrate salt, ground to a particle size of about 200 microns. 5. Paraffin wax, AW 3501 from Ter Hell Paraffine, Germany, having a particle size of about 600 microns and a softening temperature about 54 to 58°C. 6. Paraffin oil, having a melting point of about -18°C. 7. Wetting agent, SAA from Nippon Oil and Fats.

Claims: An explosive composition in semi-plastic, paste or slurry form, comprising more than 10 percent by weight of at least one liquid nitric acid ester compound, gelled with nitrocellulose, at least one crystalline pulverulent component comprising an oxidizing salt, and a flegmatizing agent, characterized in that said flegmatizing agent comprises 0.5 to 10 percent, by volume of the composition, of solid particles of a material more soft or deformable than the particles of said crystalline pulverulent component, and in that the flegmatizing particles have a weight average particle size greater than the weight average particle size of the crystalline pulverulent component. The explosive composition of claim 1, wherein the liquid nitric acid ester compound is selected from nitroglycerine and nitroglycol. The explosive composition of claim 1, wherein the flegmatizing particles have a melting or softening temperature above the intended use temperature for the explosive composition before initiation, preferably above 50°C. The explosive composition of claim 1, wherein the flegmatizing particles have a Moh hardness below 1. The explosive composition of claim 1 wherein the flegmatizing particles have a particle size greater than 2 times the particle size of the crystalline pulverulent component. The explosive composition of claim 1, wherein the flegmatizing agent is a wax, preferably a petroleum wax. The explosive of claim 1, wherein the volume amount of flegmatizing agent is less than the amount of the crystalline pulverulent component in the explosive composition.

Inventors: EGELI OYVIND, EGELI, OYVIND
Applicants: DYNO INDUSTRIER AS, DYNO INDUSTRIER A/S
Application year: 1994
Main classification: C06B23
IPC codes: C06B23